Thanks!

Interestingly I can't find any such mention in the database of drugs sold locally. Closest I've seen was interaction warning for methylphenidate with alpha2 agonist medication. Which caffeine isn't but such agonist would have indirect effect on adenosine which is the same thing caffeine primarily affects. But the warning wasn't about effectiveness but about safety.

I'll have to look up actual studies on it later. If I remember. :-)

What was the constant warning across stimulant medications that I've seen is about anything which affects blood pressure and heart rate, as safety concern. And caffeine would fall under that given sufficient dosage.